1. Field of the Invention
This invention relates to a novel catalytic cracking process to produce motor fuels. In particular, this invention relates to an improved catalytic cracking process involving the use of an admixture of AgHZSM-5 and a conventional cracking catalyst to increase gasoline octane number and total yield.
2. Description of the Prior Art
Hydrocarbon conversion processes utilizing crystalline zeolites have been the subject of extensive investigation during recent years, as is obvious from both the patent and scientific literature. Crystalline zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes including the catalytic cracking of a gas oil to produce motor fuels and have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. It is also known in the prior art to incorporate the crystalline zeolite into a matrix for catalytic cracking and such disclosure appears in one or more of the above-identified United States patents.
It is also known that improved results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 Angstrom units is included with a crystalline zeolite having a pore size greater than 8 Angstrom units, either with or without a matrix. A disclosure of this type is found in U.S. Pat. No. 3,769,202. Although the incorporation of a crystalline zeolite having a pore size of less than 7 Angstrom units into a catalyst composite comprising a larger pore size crystalline zeolite (pore size greater than 8 Angstrom units) has indeed been very effective with respect to raising of octane number, nevertheless it did so at the expense of the overall yield of gasoline.
Improved results in catalytic cracking with respect to both octane number and overall yield were achieved in U.S. Pat. No. 3,758,403. In said patent, the cracking catalyst was comprised of a large pore size crystalline zeolite (pore size greater than 7 Angstrom units) in admixture with ZSM-5 type zeolite wherein the ratio of ZSM-5 type zeolite to large pore size crystalline zeolite was in the range of 1:10 to 3:1.
The use of ZSM-5 type zeolite in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of ZSM-5 type zeolite in amounts up to and about 5 to 10 weight percent; the latter patent discloses the weight ratio of ZSM-5 type zeolite to large pore size crystalline zeolite within the range of 1:10 to 3:1.
The addition of a separate additive catalyst comprising one or more members of the ZSM-5 type has been found to be extremely efficient as an octane and total yield improver when used in very small amounts in conjunction with a conventional cracking catalyst. Thus, in U.S. Pat. No. 4,309,279, it was found that only 0.1-0.5 weight percent of a ZSM-5 type catalyst added to a conventional cracking catalyst under conventional cracking operations could increase octane by about 1 to 3 RON+0 (research octane number without lead).
It has also become known that incorporation of silver into ZSM-5 improves the steam stability of the catalyst. Thus, under the severe hydrothermal condition often encountered during oxidative regeneration of hydrocarbon conversion catalysts or as a result that one of the products of a conversion process is water, such as during the conversion of methenol to gasoline, the catalyst will maintain catalytic activity and structure crystallinity. However, the improved gasoline selectivity and higher octane achieved with AgHZSM-5 in combination with conventional cracking catalysts as in the present invention is unexpected based on the prior art where addition of a metal function results in very substantial losses in gasoline product with resulting increases in gas make.
In order to reduce automobile exhaust emissions to meet federal and state pollution requirements, many automobile manufacturers have equipped the exhaust system of their vehicles with catalytic converters. Said converters contain catalysts which are poisoned by tetraethyl lead. Since tetraethyl lead has been widely used to boost the octane number of gasoline, refiners now have to turn to alternate means to improve gasoline octane number.
One method of increasing octane number is to raise the cracker reactor temperature. This method, however, is very limited, since many units are now operating at maximum temperatures due to metallurgical limitations. Raising the cracker reactor temperature also results in increased requirements for the gas plant (i.e., gas compressor and separator). Since most gas plants are now operating at maximum capacity, any increase load could not be tolerated by the present equipment.
An alternative method has been to mix an additive catalyst such as ZSM-5 to the cracking catalyst as described above. Generally, the octane gain of a ZSM-5 containing cracking catalyst is associated with gasoline (C.sub.5 +) yield decrease and correspondingly higher yields of C.sub.3 and C.sub.4 gaseous products. As the freshly added ZSM-5 undergoes hydrothermal deactivation the octane enhancement is reduced and additional ZSM-5 must be added to maintain the desired octane level.
As can well be appreciated in the foregoing, it would be extremely desirable to have a more steam stable ZSM-5 additive which would in effect reduce the additive catalyst requirement to maintain a given octane level.